From GB-A-2148487 an apparatus for optical analysis of a smoke gas is known. Said apparatus requires, however, an optically clear view, if necessary assisted by mirrors or lenses, of a detection zone through a smoke gas. The zone may be identical to the width of a duct for transporting the smoke gas in that the two nearly identical parts of the apparatus are mounted perpendicularly to the duct and the sensors contained in each part are protected against the harmful effect from the smoke gas by a protective gas fed into each part. The optical signal may vary corresponding to the gas density in the detection zone and is transferred into an electric signal to be read in a meter. The apparatus is only suited for measuring the density of a smoke gas and gives no information as to other essential properties of a gas, such as composition, homogeneity or structure. For that purpose a photoacoustic measurement of the smoke gas has to be made by means of an apparatus allowing the analyzed amount of gas to be introduced into a separate analyzing zone for further examination.
U.S. Pat. No. 3,659,452 discloses a method and an apparatus for photoacoustic spectroscopy of a gas by means of an acoustic resonance cell.
U.S. Pat. No. 4,253,770 discloses an apparatus for optoacoustic analysis by means of the light from a laser resonator and a differential cell assembly.
CH-B-344234 discloses a solely acoustic gas analyzer in which the microphone may be mounted in a side tube to the tube containing the gas sample and be protected against the harmful effects from the analyzed gas by a thin layer of inert gas. A mixing between the analyzed gas and the protecting inert gas has, however, to be considered, and the protection of the microphone is only to be ensured when the relation between the two gases is kept at 100:1. In addition, the mounting of the microphone in a side tube inevitably implies an unwanted weakening of the acoustic signal.